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west4x4



Member Since: 23 Jan 2010
Location: Rutland
Posts: 147

United Kingdom 
Batteries double or triple?
Just purchased a lithium battery to boost capacity running fridges lights etc camping. Now I already run 120aH AGM in the battery box running through a switched relay system.

The Lithium needs a different charge system which is fine but I’m thinking about the 2 under the seat should I
1) just join the 2 under the seat (they are a matching pair) in parallel. Gives massive capacity but leaving lights on could drain both. But a much simpler system
2) keep the split charge. allows use of the jump function some existing accessories could still run off second battery but extra wiring and could cause issues with rate of charge to the lithium?

Engine is a 300tdi with a standard 65amp alternator
Post #1077825 18th Oct 2025 1:07pm
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Mdm



Member Since: 11 Sep 2013
Location: Sunny Lancashire
Posts: 1688

United Kingdom 
for clarity. do you have a standard battery for the car and a split charge to run the agm?
are they both same size and spec and agm ones?

if they are the same then you can join them together minus the split charge without much issue.
if not then its not recommended.

agm need a certain input of amps and type of battery charger to get the best out of them.

lithium need a different charger set up as well.

without the lithium spec its hard to know what it will power and more so what you plan to run off it for how long.
i would have thought that 1 good battery would replace the agm with no problems.

you could also consider a larger spec std alternator and or a 2nd one , (gwyn lewis does a nice kit) sole use to power the aux battery system, this could be 100=140 amp spec as well.
Post #1077833 18th Oct 2025 2:35pm
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jst



Member Since: 14 Jan 2008
Location: Taunton
Posts: 8476

 2011 Defender 110 Puma 2.4 USW Stornoway Grey
If the main aim is extra camping current then don't parallel the first two as then only lithium for camping.

As mdm says ideally 2 different chargers needed for agm and lithium compared to std.

Personally I am running 1750agm as main battery and then 2 x 105ah leasure batteries on split charge for camping. .plus solar controller for leisure Cheers

James
110 2010 XS Utility
130 2011 M57 bespoke Camper
90 2010 Hardtop
90 M57 1988 Hardtop
Post #1077853 18th Oct 2025 7:40pm
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west4x4



Member Since: 23 Jan 2010
Location: Rutland
Posts: 147

United Kingdom 
Mdm wrote:
for clarity. do you have a standard battery for the car and a split charge to run the agm?
are they both same size and spec and agm ones?

if they are the same then you can join them together minus the split charge without much issue.
if not then its not recommended.

agm need a certain input of amps and type of battery charger to get the best out of them.

lithium need a different charger set up as well.

without the lithium spec its hard to know what it will power and more so what you plan to run off it for how long.
i would have thought that 1 good battery would replace the agm with no problems.

you could also consider a larger spec std alternator and or a 2nd one , (gwyn lewis does a nice kit) sole use to power the aux battery system, this could be 100=140 amp spec as well.


Current batteries are 2 identical 120ah AGMs with a T-Maxx split charge. Powers 2 auxiliary fuse boxes one under drivers seat powering CB and some cabin lights. One in rear running a Snomaster fridge freezer plus some more lighting. Lithium is 300AH (yes I know it’s overkil) be getting a DC-DC charger for it with some provision for solar. Adding a diesel heater and perhaps looking to run and induction hob instead of gas.

Upgraded alternator has been considered didn’t think about a second one I will investigate that.
Post #1077863 18th Oct 2025 9:38pm
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